The annular and spirally thickened conducting elements generally develop in the protoxylem when the root or stem is:

Explanation

Annular and spiral thick carrier elements usually develop in the protoxylem when the roots or stems are maturing. The maturity of the root or stem is the development of the parts so that the plant can easily become a tree. This is the time when the protoxylem develops or produces annular and spiral thick carrier elements. They also grow plants to provide mechanical support.